GL.IssuePluginEvent with multiple plugins

I'm really not clear on how IssuePluginEvent is supposed to connect to native plugins, or really get used at all. The documentation is very limited, so there's a lot of guesswork involved (it sees like most questions regarding this call in this forum go unanswered).

The main question I have is how this call is supposed to work if you have multiple plugins that need to do native rendering. In the example code for iOS, you register the UnityRenderEvent(int marker) callback when the app starts. But this call isn't specific to the native plugin I'm designing. In our case this may not be an issue since we only have one plugin that needs to do native rendering, but how is this supposed to work if multiple plugins from different sources need this ability? I don't see any obvious way of directing the "plugin event" to the right place. The eventId is very mysterious as I don't see any way to link specific calls to specific IDs.

Maybe I'm missing the whole point of this call, but it would certainly be nice if there was more documentation or examples. The sample is pretty useless.

EDIT: Just for context, I have a native plugin that made OpenGL calls that worked perfectly well in Unity 4. It no longer works in Unity 5 and I have no idea why, so I'm trying to get to the bottom of it.